A conventional linear actuator is shown in FIGS. 4 and 5 of Japanese Patent Application Laid-Open No. 7-332454 Publication. In the linear actuator of this kind, when a driving force and a braking force of its motor disappears, and a tension or a pressing force is exerted on an output shaft (not shown), the tension or the pressing force generates a rotating force in the ball-threaded shaft which has a thrust bearing assembly mounting the shaft in a housing.
Any axial tension exerted on the ball-threaded shaft tends to displace the bearing assembly, and such displacement is prevented by a tension stopper spread and inserted into a central annular groove in the inner periphery of the housing. The assembly includes an axial disk, a first thrust bearing, a first tension receiving rotational body and a second tension receiving rotational body. When the tension exerts a force on the tension stopper, the second tension receiving rotational body is restrained in rotation by the frictional force with respect to the tension stopper, and the first tension receiving rotational is likewise restrained in rotation by the frictional force with respect to the second tension receiving rotational body. Further, a coil spring is frictionally engaged between the axial disk and the first body whereby the axial disk is restrained in rotation with respect to the first tension receiving rotational body. Accordingly, the ball-threaded shaft is prevented from being rotated.
On the other hand, any axial pressing force exerted on the ball-threaded shaft tends to displace the bearing assembly against a pressing force stopper secured to the end of the housing. The displacement is prevented by the axial disk, a second thrust bearing, and a pressing force receiving rotational body. The ball-threaded shaft is prevented from being rotated when under a pressing force similar to the aforementioned restraint when under tension.
However, in the conventional linear actuator, when the driving force or the braking force of the motor is interrupted, a rotating force is exerted on the ball-threaded shaft. This rotating force is transferred to the tension stopper through the axial disk, the coil spring, the first tension receiving rotational body and the second tension receiving rotational body. Since a frictional force exists between the tension receiving rotational body and the tension stopper, the C-shaped tension stopper tends to rotate and become reduced in diameter by the rotating force. Since the ball-threaded shaft is in under tension, when the reduced-diameter C-shaped tension stopper moves out of the central annular groove of the housing, the bearing assembly may slip out of the housing past the central groove. Accordingly, in the case where the output shaft suspends a heavy article, there poses a problem in that the heavy article drops, resulting in a very dangerous state.